1. Field of the Invention
The present invention relates to a semiconductor apparatus capable of preventing electrostatic discharge damage.
2. Description of Related Art
With an increase in the degree of integration and the speed of semiconductor apparatus, electrostatic discharge damage that is caused by electrostatic discharge (ESD) which occurs when handling semiconductor apparatus has emerged as a significant problem. A resistance to electrostatic discharge damage which can cause breakdown of semiconductor apparatus decreases as a device becomes smaller and a high-speed but fragile semiconductor material/structure is introduced. An example of the high-speed semiconductor material and structure is a SiGe heterojunction bipolar transistor, which uses SiGe for a base region to reduce a band gap of the base region to be smaller than a band gap of an emitter region, thereby increasing the emitter injection efficiency to improve high-frequency characteristics. Another example is a SiGe:C heterojunction bipolar transistor, which adds Carbon (C) to the SiGe base for the purpose of reducing lattice distortion due to Ge and suppressing diffusion of Boron (B) to obtain a shallow and high-concentration base.
An electrostatic protective device having the PN junction with N-type Si and P-type Si has been used. FIG. 11 shows a protective transistor using an NPN Si bipolar transistor which is disclosed in Japanese Unexamined Patent Publication No. 10-200056. The emitter of the protective transistor 105 is connected with a terminal 102 to be protected from electrostatic discharge damage. The base and the collector of the protective transistor 105 are short-circuited and connected with a terminal 103 to discharge static electricity. In this example, a circuit to be protected is a Si bipolar transistor 104, which has the base connected with the terminal 102 and the emitter connected with the discharge terminal 103. This structure aims at protection from electrostatic discharge damage when static electricity is applied between the base and the emitter of the Si bipolar transistor 104. If positive static electricity is applied to the base of the NPN Si bipolar transistor, forward current flows between the base and the emitter, which rarely causes breakdown. On the other hand, if negative static electricity is applied, a reverse bias is established and no current flows between the base and the emitter, in which case a voltage can reach a breakdown limit. With the use of the protective transistor 105 having the N—Si emitter connected with the terminal 102 and the P—Si base connected with the discharge terminal 103, a forward bias is applied to the protective transistor 105 upon application of negative static electricity so that negative charges are discharged through the emitter and the base, thereby protecting the bipolar transistor from breakdown.
FIG. 12 shows an example of using a PN Si diode 106 instead of the NPN Si bipolar transistor 105. The cathode of the PN Si diode 106 is connected with the terminal 102 to be protected from electrostatic discharge damage, and the anode of the PN Si diode 106 is connected with the terminal 103 to discharge static electricity. Because the cathode is N-type Si and the anode is P-type Si, this structure can also protect the bipolar transistor 104 in the same manner as the structure using the NPN Si bipolar transistor.
FIG. 13 shows an example of an electrostatic protective device using a SiGe bipolar transistor which is disclosed in Japanese Unexamined Patent Publication No. 2002-313799. An external ESD protective device 111 which is connected with an input/output pad 112 of a circuit 113 to be protected, an external resistor 110, and an ESD protective circuit 107 are connected in series. The ESD protective circuit 107 includes an internal resistor 109, an internal capacitor 108 and a SiGe bipolar transistor having a collector and an emitter connected with a power supply voltage VDD and VSS, respectively. The external ESD protective device is composed of a SiGe Schottky diode, a SiGe transistor, a SiGe varistor, or a SiGe diode.
However, the electrostatic protective device using the Si PN junction and the electrostatic protective device using the SiGe bipolar transistor disclosed in Japanese Unexamined Patent Publication No. 2002-313799 have the following problems.
With the use of the electrostatic protective device using the Si PN junction, a Fermi level is close to the lower limit of a conduction band in N-type and it is close to the upper limit of a valence band in P-type as shown in a band width diagram of PN junction in FIG. 9. Therefore, a built-in potential is determined by the band gap of Si, and the ON-state voltage at which current starts flowing upon application of a forward voltage is 1.1V or higher. Thus, when static electricity is applied to a terminal to be protected, no current flows to an electrostatic protective device until reaching the ON-state voltage, which results in a low electrostatic discharge resistance.
With the use of the electrostatic protective device using the SiGe bipolar transistor disclosed in Japanese Unexamined Patent Publication No. 2002-313799, the external resistor 110 and the internal resistor 109 are connected in series through a discharge path. Thus, when static electricity is applied to a terminal to be protected and current starts flowing, voltage drop occurs in the resistors and a voltage is not sufficiently applied to the PN junction to cause lower discharge current, which also results in a low electrostatic discharge resistance.